In a device that receives a communication signal or a broadcasting signal, a decision feedback equalizer that performs a waveform equalization nonlinearly is widely used for reducing influence of an intersymbol interference in received signals. FIG. 7 illustrates a decision feedback equalizer 1 described in Non-patent Document 1 (“Digital Communication”, John G. Proakis, Kagaku Gijutsu Shuppan, p752-753) and Patent Document 1 (JP 2001-177451 A). The decision feedback equalizer 1 includes a plurality of weighting devices 2a-2e. The weighting device 2a at a first stage directly receives an input signal (tap a), the weighting device 2b receives the input signal through a delay element 3a (tap b), and the weighting device 2c receives the input signal through the delay element 3a and a delay element 3b (tap c).
Each of the weighting devices 2a-2e includes two multipliers 4, 5 and an accumulator 6. The multiplier 4 multiply the input signal by a tap coefficient and outputs a multiplication result to the accumulator 6. The multiplier 5 multiplies the input signal by an output signal of the accumulator 6 and outputs a multiplication result to the outside. An adder 7 adds output results of the weighting devices 2a-2e and outputs an addition result to a detector 8. The detector 8 performs a symbol detection to the input signal and outputs a data value±1. In other words, the detector 8 outputs a non-return to zero (NRZ) signal.
The output signal of the detector 8 is input to the weighting device 2d through a delay element 3c (tap d), and is further input to the weighting device 2e through a delay element 3d (tap e). A subtractor 9 calculates a difference between the input signal and the output signal of the detector 8 and outputs the difference to a tap coefficient updating portion 10. The tap coefficient updating portion 10 updates the tap coefficient by multiplying the difference by an update coefficient Δ and outputs the updated tap coefficient to the multipliers 4 in the weighting devices 2a-2e. The weighting devices 2a-2c are feed forward (FF) side weighting devices, and the weighting devices 2d, 2e are feedback (FB) side weighting devices.
In general, in order to improve a convergence accuracy of a decision feedback equalizer, it is necessary to increase the number of taps so as to increase a resolution. However, when the decision feedback equalizer 1 having the above-described configuration is actually formed into an integrated circuit, it is necessary to consider timing of signals transmitted between elements. If the number of taps in the decision feedback equalizer is increased, the number of fan-out of the tap coefficient updating portion 10, which updates the tap coefficient of all of the weighting devices 2a-2e, increases, and an increase amount of a delay time becomes a value that is not negligible, for example, by influence of a stray capacitance. Thus, it is necessary to dispose a delay element between elements forming the decision feedback equalizer 1 so as to satisfy a timing limitation (or a driving ability).
In other words, as illustrated in FIG. 8, in each of the weighting devices 2a-2e in a decision feedback equalizer 14, delay elements 11a, 11b are disposed on an input side of the multiplier 4, and a delay element 11c is disposed on an output side of the multiplier 4. In addition, between each of the weighting devices 2a-2e and the adder 7, delay elements 12a-12e are disposed, and a delay element 13 is disposed on an output side of the adder 7.
In the decision feedback equalizer 14, delays of feedback paths of updating the tap coefficients are equal among the respective taps (the delay unit number “2”). In addition, the delay on the input side of the multiplier 4 is set to be equal to the delay of the feedback path. The delay element 11c between the multiplier 4 and the accumulator 6 is disposed so as to compensate for a driving ability of an output signal of the multiplier 4.
However, when the decision feedback equalizer 14 is actually formed, the update timing of the tap coefficient may delay by influence of the delay elements 12a-12e, 13, a calculation may be performed using old information (tap coefficient before updating), and a convergence accuracy (waveform equalization operation) of a receive signal waveform may be deteriorated. As a result, following characteristics of the decision feedback equalizer 14 may be deteriorated when characteristics of a communication channel change.